1. Field of the Invention
The present invention relates to the preparation of aromatic aldehydes, and, more especially, to the preparation of aromatic aldehydes by the hydrocarbonylation of aromatic halides in the presence of a noble metal, a tertiary nitrogenous base and, where necessary, a phosphorus compound.
2. Description of the Prior Art
A process for the preparation of aldehydes by reacting a gaseous mixture of hydrogen and carbon monoxide with an organic halide of the group consisting of aromatic, vinyl and heterocyclic halides in the presence of a tertiary amine and a palladium catalyst which consists of a complex of a divalent palladium derivative with a phosphine (triphenylphosphine), a phosphite or an arsine, or of the combination of a divalent palladium salt (acetate or chloride) or finely divided palladium metal with a phosphine, phosphite or arsine complexing agent, is described in U.S. Pat. No. 3,960,932. In the latter case, the ratio of the number of g-at. of P to the number of g-at. of Pd may be between 0.5 and 5. The reaction is carried out at a temperature of 75.degree. C. to 175.degree. C. and at a pressure of 7 to 140 bar. In general, the nitrogenous base is used in slight excess compared with the theoretical amount required for the neutralization of the hydracid which is a byproduct of the reaction. This process is particularly well suited for the preparation of aromatic aldehydes by the hydrocarbonylation of the corresponding bromides. In spite of the good results obtained, this process suffers from a serious disadvantage in the time periods required for reaction, i.e., periods on the order of 10 to 26 hours. These reaction times result in low productivities of the equipment and negate any industrial value of the process.
In order to alleviate the disadvantages of the process described in U.S. Pat. No. 3,960,932, the hydrocarbonylation of aromatic halides at pressures of 20 to 400 bar, at a temperature of 80.degree. to 250.degree. C., in the presence of a noble metal-based catalyst, a tertiary nitrogenous base and a large amount of a phosphine or a phosphite has been proposed. See published European Patent Application No. 0,109,606. The amount of the phosphorus derivative represents from 2 to 10.sup.5 times the molar amount of the catalyst, preferably from 10 to 1000 times. As it enables the use of high reaction temperatures without degradation of the catalyst, this particular process enables the reaction rate, and, consequently, the productivity of the equipment to be increased. However, the increase in the reaction rate is still considered insufficient and inexorably linked to the use of high temperatures.
In the processes of the prior art, the highest possible pressures have been used (in the '606 European application, it is noted that the total pressure must be at least 20 bar) and the CO/H.sub.2 ratio in the gaseous mixture has not been considered critical; this ratio is preferably equal to 1.
Cf. Schoenberg et al, Journal American Chemical Society. 96, No. 25, pp. 7761-7764 (Dec. 11, 1974); EP-A-No. 0,165,881.